Conventional CMOS image sensors can have a linear response to relatively low illumination and a logarithmic response to relatively high illumination, with a transition between the linear and logarithmic responses being referred to as a knee point (KNPT). As will be understood by those skilled in the art, an image sensor may be fully characterized when its linear sensitivity, logarithmic sensitivity (LOGS) and KNPT voltage are known. Because these parameters vary with process, transistor mismatch and temperature, etc., calibration is required. Without accurate calibration, a linear-logarithmic image sensor may introduce very large fixed pattern noise (FPN), which is typically not suitable for commercially viable applications. One conventional technique to inhibit FPN uses an external frame memory to store a sensor characteristic response. Based on this technique, a correction of the sensor readout for each pixel is performed after a full frame of image data is captured by an image signal processing (ISP) circuit and then modified by the sensor data stored in the external frame memory, which may have a relatively large capacity. Moreover, relatively complex ISP algorithms may be required to perform image sensor correction, which can be computationally expensive and time consuming.
A conventional linear-logarithmic image sensor is disclosed in U.S. Pat. No. 7,804,537 to Storm et al., entitled “Combined Linear-Logarithmic Image Sensor.” This sensor includes an array of pixels, with each pixel including a photodiode and a first output circuit for deriving a linear output signal by applying a reset signal to a photodiode and reading a voltage on the photodiode after an integration time. A second output circuit is also provided, which derives a logarithmic output signal by reading a near instantaneous illumination-dependent voltage on the photodiode that is a logarithmic function of the illumination. In the logarithmic mode, the pixels are calibrated to remove fixed pattern noise (FPN). The pixels may be operated in linear and log modes sequentially with the linear output being selected for low light signals and the log output being selected for high light signals.